SDK / exoplayer

Currently onTimelineChanged doesn't allow to distinguish easily between the
different reasons why it's being called. Especially, finding
out whether a new media source has been prepared or the current source
refreshed dynamically was impossible without tightly coupling the player
operations with the listener.

The new reasons provide this disdinction by either indicating a newly
initialized media source, a dynamic update to an existing timeline
or manifest, or a reset of the player (which usually results in an
empty timeline).

The original onTimelineChanged method without reason is kept in the
DefaultEventListener as deprecated to prevent the need to update all
existing listeners in one go.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176478701